A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning structure, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scamming the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning structure to the substrate by imprinting the pattern onto the substrate.
The application of gas showers is known from the art. For example, European patent EP 0 498 499 illustrates, in FIG. 18 thereof, a part of a lithographic apparatus. The apparatus includes an interferometer system and a space in which the interferometer beams propagate. A constant, preferably laminar, stream of air is passed through this space to obtain a greater accuracy of the interferometer system. Both the purity and the temperature of the supplied air may be controlled. The air may be, for example, of purity class 1 and its temperature may be, for example, stable within 0.10 degrees C.